The basic mechanic of Ego Whip is to overwhelm the other person's mental apparatus. In the best case scenario, you can actually take control of their mind. The shock of the attempted takeover might also cause insanity, or at least knock them out of commission for awhile.
The Ego Whip costs 7 attack points against a psionic, or 27 points versus a non-psionic. Remember, a psionic must have at least 100 strength points to attack a non-psionic. The Ego Whip affects only one creature, and its range is S = 4", M = 8", and L = 12".
Note that mind blank is normally helpful against psionic attacks, but against the Ego Whip, mind blank is actually harmful. According to the DMG psionic combat tables, Ego Whip is twice as effective against Mind Blank as any other psionic defense. Because Ego Whip is basically trying to highjack the victim's thought processes, the Mind Blank actually helps things along. It's like a "lay down" defense against a grappling attack. Yeah, not too smart.
Savings throw table